we have a standard IIS SMTP server installed. One of its sending default accounts is Network Service.  Reporting Services has to be configured to use it.

Open the Reporting Services Configuration Manager and go to Service Account. Change it to Network Service.  You will be prompted to save an encryption key with a password.

Go to Email Settings. Put in the sending email address, Use SMTP server is chosen for you, and the SMTP Server ip is 127.0.0.1

Go to the root of the tree on the left, stop and restart the service.

You’re going to need this for Subscription Services. On the SMTP server in iis choose properties, delivery outbound relay and uncheck limit connections to.   My emails kept getting queued for failing to do so.